Use 'i' as parameter name not 'I'.

The latter causes problems when complex.h is #included.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7233)

(cherry picked from commit 972f67889b5a10c33dbc3d500cfa65b544e68b46)